As per my comments, I think making a simple HTML page to serve as help documentation would be the easiest solution. Python Add Ins are fairly limited with pop up options. The HTML page wouldn't need to be hosted on a web server and can just be included in the
install directory of the Add In and use relative paths to open this when the
onClick() method is invoked.
The HTML doc can easily be opened using the builtin webbrowser module. So if you have a structure like this:
You can open your
help.html file in a web browser by adding this to your